Flexible High Power Microwave Comb Generation
by Joel Libove, Ph.D., President, Ultraview Corp.
Palm-Sized Differential Pulse/Comb Generator with USB-programmable Spectral Energy Weighting and 10 MHz to 2 GHz Picket Spacing Enables New Test Capabilities
RF/Microwave comb and pulse generators are used for testing broadband antennas, RF devices and subsystems, as well as being used as transmitters for radar applications and medical imaging systems, and for driving lasers and optical modulators. Comb generators have traditionally been bench-top instruments which required long, lossy cables to connect their output to an antenna or device under test (DUT). Alternatively, compact coaxial comb generators are available that can be located at the DUT, but they have limited output power, must be driven by an external RF source, require a BALUN to drive differential loads, and operate only over narrow pulse repetition rate and pulse width ranges. A novel GaN variable width, variable pulse repetition rate differential comb generator, the Ultraview Ultracomb-8G (Figure 1), overcomes all of these limitations, providing previously unattainable RF output power levels and producing combs with an unprecedented 2000:1 range of picket spacings (spacing between spectral peaks). Based on a custom ultra-high repetition rate GaN differential pulser IC based on the Dynamic Cascode Exchange (DCE) pulse generation architecture, the palm-sized Ultracomb-8G is powered from any USB3.0/3.1 port through which the user can program comb amplitude, picket spacing and even the spectral weighting of the entire comb—all in real time.
Ultra-flexible Ultrawideband (UWB) Comb Generation
Comb picket spacing can be software-programmed to any frequency from 10 MHz to 2 GHz in single-ended-output mode (10-50 MHz in differential output mode) in 0.01 Hz steps, generated by an onboard low phase noise LMX2594 synthesizer driven from either an internal 150fs-jitter reference clock or a 10-500 MHz external reference. The unit can also be programmed in 1:1 clock mode, enabling the pulse repetition rate (which is also the comb picket spacing) to be the same as the external reference/clock input. Its differential outputs provide a 3dB higher total output power than a single-ended pulse generator, and facilitate direct antenna connections without the use of an expensive and large broadband BALUN.
Very High Output Power
At the narrowest pulse width setting, and with a 500 Million Pulse Per Second (500 MPPS) pulse repetition frequency (500 MHz picket spacing), each generated picket up to 3 GHz has over +10dBm power level. Even up to 6 GHz, each picket has over 0dBm power, as shown in Figure 2. When the unit is generating a comb with a finer 200MHz picket spacing it produces flat response with 0dBm/picket power to 3 GHz, -3dBm at 5.4 GHz and -10dBm at 8 GHz. For higher power, the picket spacing can be increased. Figure 3 shows a 1250MPPs pulse repetition rate (and hence 1250 MHz picket spacing), resulting in the first picket being at +16dBM, and the 8.4 and 9.6 GHz pickets being at -2dBM. The higher per picket power at higher pulse repetition rates is the direct result of the generator producing a higher number of pulses per second, and hence a higher aggregate power level.
Programmable Pulse Width and Corresponding Low/High Spectral Weighting
The Ultracomb-8G has the further unique ability to create strings of pulses with widths varying from 100ps to 800ps, enabling it to generate relatively flat combs with usable energy to 10 GHz or, alternatively, combs with much higher power, but with most of the energy concentrated below 2 GHz. This is useful, for example, in antenna testing, in which low frequency antennas can be tested over very long ranges or transmitting through lossy media, while retaining the ability to test over a wider bandwidth when using shorter ranges. Figure 4 shows the generation of precisely matched differential pulses of 152ps FWHM (top blue traces) and then, by software selection, a 350ps FWHM differential pulse (yellow and green traces, shown at 200ps/div in the top window, and 1ns/div in the bottom window).
Cross-Platform Operability for Flexible Field Use Environment
The palm size of the unit enables its use in the field, directly connected to antennas or devices using low-loss cables of only a few centimeters in length. Virtually any desktop or laptop computer can be used as the GUI software is included for Windows 10, OSX, Linux Mint 18 and RHEL/Centos 7.x. In addition to the GUI, full QT source is supplied, enabling users to dynamically control the comb amplitude, picket spacing, and frequency weighting in real time as part of a larger field or laboratory environment.
Previous Pulse/Comb Generation Methods vs Dynamic Cascode Exchange (DCE) Method
Conventional pulse/comb generators are usually based on step recovery diodes (SRDs), avalanche transistors, or non-linear-transmission lines (NLTLs). An SRD is generally driven by a high power sine or square wave signal alternately into forward bias and then into reverse bias. During forward bias, the PN junction is infused with minority carriers. When the SRD is then driven into negative bias, the minority carriers will exit the junction and when they are reduced to near zero, the reverse current will very suddenly terminate, generating a sharp step voltage that can be differentiated using passive circuitry, into a pulse. The time required to inject and deplete carriers from the junction limits the frequency range to about 100MPPS (higher frequencies are attainable using resonant circuits to increase drive). SRDs also have an order of magnitude higher jitter than DCE pulsers, and do not have a wide range of adjustability of step edge rate or amplitude. Alternatively, avalanche transistors can be used in pulse generators, as they produce sharp edges when they are biased and then trigger such that a controlled avalanche breakdown occurs between the collector and emitter terminals. They can produce higher voltages, but are again limited by the time it takes to build up the collector-emitter voltage. They also exhibit higher jitter than DCE pulsers, and do not have a wide range of edge rate adjustability. A third conventional method of pulse generation is the NLTL, which uses a string of varactors along a transmission line to generate an ever-sharpening edge as a wave propagates along the line. NLTLs have very low jitter and higher speed than DCE pulsers, but do not have dynamically adjustable pulse edge rates or the ability to produce as much output power, and they are difficult to fabricate on monolithic GaN ICs.
In the Ultracomb-8GB, DCE pulse generators are used which employ a current steering architecture (see U.S. Patent 6,433,720 for more detail) in which source-coupled (differentially connected) GaN FET transistor pairs are stacked up vertically in the circuit, so that when a single downward going edge of an input control/trigger signal occurs, current steering in the top pair produces the start of the pulse and as the control signal edge goes further downwards in voltage, a few tens of picoseconds later, current steering in the bottom pair produces the end of the pulse. The source terminals of the transistors in the bottom differential pair are fed by an adjustable current sink whose settings determine the amplitude of the generated pulse. The slew rate of the input control/trigger signal can similarly be adjusted by varying a current source to the control signal input amplifier, such that when the current is set lower, the control/trigger signal has slower edges, and the pulse generator produces a pulse with a slower rise and fall time. The circuit in patent 6,433,720 describes the basic DCE pulser concept, although the IC recently developed for the Ultracomb-8G contains enhancements to the circuitry described in the patent, so as to generate fully differential pulses.
The GaN IC used to generate microwave pulse trains and corresponding combs for the Ultracomb-8G was initially developed for radar pulse-based structural and vascular brain imaging systems (see U.S. patent number 10,660,531). The ability to make pulses of varying widths for the pulse/comb generator was directly driven by the real-world need to make wide pulses that could penetrate to image deep tissues in the human body with reduced spatial resolution, while retaining the capability to be programmed to generate narrow pulses for imaging shallower tissues with higher resolution.
Low Size, Weight and Power (SWAP) and Cost
The Ultracomb-80G weighs 13 ounces, including 2M USB cable, measures 7cm x 14cm x 3.2cm and draws 1.3A max from the USB socket. It costs $3,495 in single quantity and $2995 in quantities of 2 or more.
About the Author
Joel Libove received a BSEE from Cornell University and a M.S. and Ph.D. from University of California Berkeley. He has designed approximately 200 board level products, 65 ICs, and holds 18 patents, including a recent patent for a microwave vascular and functional brain imager.